1. Field of the Invention
The present invention generally relates to a method of optimizing an imaging performance of a projection exposure system and a projection exposure system adapted to carry out the method.
2. Brief Description of Related Art
Lithographic processes are commonly used in the manufacture of semiconductor elements, such as integrated circuits (ICs), LSIs, liquid crystal elements, micropatterned members and micromechanical components.
A projection exposure apparatus used for photolithography generally comprises an illumination optical system with a light source and a projection optical system. Light from the illumination optical system illuminates a reticle (mask) having a given pattern and the projection optical system transfers an image of the reticle pattern onto a photo-sensitive substrate. The image of the reticle pattern may also be reduced in size by the projection optical system so as to produce a smaller image of the reticle pattern on the substrate.
The trend towards ever more sophisticated semiconductor devices requires semiconductor elements of smaller size and higher complexity which, in turn, makes higher demands on an imaging performance achievable with projection exposure systems. In particular, the image of the reticle pattern needs to be very accurately projected onto the substrate. Therefore, for instance, aberrations of the wave-fronts of the light transferring the image of the reticle pattern onto the substrate need to be reduced to the greatest extent possible. Also other factors, such as transmission properties, evenness and efficiency of illumination as well as polarization effects need to be closely controlled.
In addition, efforts are being made to increase the throughput of exposures such as to decrease the overall manufacturing time and thus manufacturing costs. The efficiency of the lithographic process therefore needs to be optimized.
Various methods and systems are known which allow to alter the imaging performance of the projection exposure systems after assembly of the various components comprised in the projection exposure system. For instance, one or more lenses can be tilted or moved perpendicular to or along an optical axis of the system to reduce certain types of aberrations, such as astigmatism.
Generally, a projection exposure system is optimized after assembly to provide optimum performance.
However, after assembly and optimization, the imaging performance of a projection exposure system may undergo a change induced by a variation in atmospheric conditions of the environment the projection exposure system is exposed to, for example. Transport of the assembled projection exposure system may also cause mechanical changes affecting a quality of a projection exposure. The imaging performance of the projection exposure system may also change over time due to damage to optical elements induced by extended exposure to radiation and resulting change in aberrations, transmittance and/or reflectance, for instance. In addition, a change of a mode of illumination, for instance off-axis illumination or multipole illumination modes, may require imaging characteristics to be adapted.
Given the ever increasing demands on imaging performance and efficiency, there remains a need for a method of optimizing an imaging performance of a projection optical system and projection exposure systems adapted to carry out the method.